Fuel pump main drive shafts typically incorporate a spline at each end. The spline at the outboard end is connected to the engine gearbox while the spline at the inboard end is connected to a rotating pump component. While the spline attached to the engine gearbox is lubricated by oil, the spline at the other end is lubricated by less lubricous fuel, thereby resulting in unequal spline wear. The basic problem is that observation of spline wear at the readily accessible outboard end of the shaft provides no indication of spline wear at the inaccessible inboard end, whereby actual inspection is necessary to ascertain the extent of wear at the inboard end.